A Nd—Fe—B based sintered magnet (so-called neodymium magnet) is made of a combination of iron and elements of Nd and B that are inexpensive, abundant, and stably obtainable natural resources and can thus be manufactured at a low cost and additionally has high magnetic properties (its maximum energy product is about 10 times that of ferritic magnet). Accordingly, the Nd—Fe—B sintered magnets have been used in various kinds of articles such as electronic devices and have recently come to be adopted in motors and electric generators for hybrid cars.
On the other hand, since the Curie temperature of the above-described sintered magnet is as low as about 300° C., there is a problem in that the Nd—Fe—B sintered magnet sometimes rises in temperature beyond a predetermined temperature depending on the circumstances of service of the product to be employed and therefore that it will be demagnetized by heat when heated beyond the predetermined temperature. In using the above-described sintered magnet in a desired product, there are cases where the sintered magnet must be fabricated into a predetermined shape. There is then another problem in that this fabrication gives rise to defects (cracks and the like) and strains to the grains of the sintered magnet, resulting in a remarkable deterioration in the magnetic properties.
Therefore, when the Nd—Fe—B sintered magnet is obtained, it is considered to add Dy and Tb which largely improve the grain magnetic anisotropy of principal phase because they have magnetic anisotropy of 4f-electron larger than that of Nd and because they have a negative Stevens factor similar to Nd. However, since Dy and Tb take a ferrimagnetism structure having a spin orientation negative to that of Nd in the crystal lattice of the principal phase, the strength of magnetic field, accordingly the maximum energy product exhibiting the magnetic properties is extremely reduced.
In order to solve this kind of problem, it has been proposed: to form a film of Dy and Tb to a predetermined thickness (to be formed in a film thickness of above 3 μm depending on the volume of the magnet) over the entire surface of the Nd—Fe—B sintered magnet; then to execute heat treatment at a predetermined temperature; and to thereby homogeneously diffuse the Dy and Tb that have been deposited (formed into thin film) on the surface into the grain boundary phase of the magnet (see non-patent document 1).
The permanent magnet manufactured in the above-described method has an advantage in that: because Dy and Tb diffused into the grain boundary phase improve the grain magnetic anisotropy of each of the grain surfaces, the nucleation type of coercive force generation mechanism is strengthened; as a result, the coercive force is dramatically improved; and the maximum energy product will hardly be lost (it is reported in non-patent document 1 that a magnet can be obtained having a performance, e.g., of the remanent flux density: 14.5 kG (1.45 T), maximum energy product: 50 MGOe (400 kJ/m3), and coercive force: 23 kOe (3 MA/m)).
By the way, as an example of method of manufacturing Nd—Fe—B based sintered magnet, there is known a powder metallurgy process. In this method, first, Nd, Fe, and B are formulated in a predetermined composition ratio, melted, and cast to thereby manufacture an alloy raw material, which is once coarsely ground by a hydrogen grinding step, and then subsequently finely ground by, e.g., jet mill fine grinding sep to thereby obtain alloy raw meal powder. Then, the obtained alloy raw meal powder is oriented in magnetic field (alignment in magnetic field), and is compression-molded in a state in which the magnetic field is being charged, thereby obtaining a molded body. This molded body is then sintered under predetermined conditions to thereby manufacture a sintered magnet.
As a compression molding method in the magnetic field, there is generally used a uniaxial pressurizing type of compression molding machine. This compression molding machine is so arranged that alloy raw meal powder is filled into a cavity formed in a penetration hole in a die, and a compression (pressing) force is applied from both upper and lower directions by a pair of upper and lower punches to thereby form the alloy raw meal powder. At the time of compression molding by a pair of punches, due to friction among the alloy raw meal powder that is filled into the cavity and due to friction between the alloy raw meal powder and the wall surfaces of the die that is set in position in the punch, a high orientation cannot be obtained, resulting in a problem in that the magnetic properties cannot be improved.
As a solution, it is known to add to the obtained alloy raw meal powder a lubricant such as zinc stearate. In this manner, by securing flowability of the alloy raw meal powder at the time of compression molding in the magnetic field, the orientation is improved and also mold releasing from the die is facilitated (see non-patent document 2).    [Non-patent document 1] Improvement of coercivity on thin Nd2Fe14B sintered permanent magnets (by Pak Kite of Tohoku University Doctor Thesis, Mar. 23, 2000)    [Non-patent document 2] JP-A-2004-6761 (see, e.g., the description of the column of prior art)